1. Field of the Invention
The present invention relates to a method of optically converting, with an optical wavelength converter device, the frequencies of two fundamental waves emitted from respective sources into a frequency which is the sum of the frequencies of the fundamental waves, and a laser-diode-pumped solid-state laser which comprises a solid-state laser rod pumped by a semiconductor laser (laser diode), more particularly, a laser-diode-pumped solid-state laser which includes an optical wavelength converter device, disposed in a resonator, for converting the wavelengths of the frequencies of a laser beam which is oscillated by a solid-state laser rod and another laser beam into a frequency which is the sum of the wavelengths of the laser beams.
2. Description of the Prior Art
There have heretofore been made various attempts to apply two fundamental waves having different wavelengths .lambda..sub.1, .lambda..sub.2 to a nonlinear optical material to extract a wave having a frequency which is the sum of the frequencies of the fundamental waves, i.e., a wavelength .lambda..sub.3 which is expressed by: EQU 1/.lambda..sub.3 =1/.lambda..sub.1 +1/.lambda..sub.2.
One well-known optical wavelength converter device which effects such optical wavelength conversion is a bulk-crystal-type wavelength converter device. Yao et al. describe a method of achieving phase matching at the time of generating a second harmonic wave with KTP which is a biaxial crystal (see Japan Applied Physics Vol. 55, page 65 (1984)).
Nonlinear optical materials which have conventionally been used in bulk-crystal-type optical wavelength converter devices include inorganic materials such as LiNbO.sub.3 and KTP, and organic materials such as MNA (2-methyl-4-nitroaniline) disclosed in Japanese Unexamined Patent Publication No. 60(1985)-250334 and NPP (N-(4-nitrophenyl)-L-prolinol), NPAN (N-(4-nitrophenyl)-N-methylaminoaceto-nitrile), etc. disclosed in Japan Optical Society Am. B. The organic optical materials are more advantageous than the inorganic optical materials because they have a higher wavelength conversion efficiency due to a larger nonlinear optical constant, a higher dielectric breakdown voltage threshold, and is less susceptible to optical damage.
The organic optical materials have absorption edges near 450 nm for MNA and 480 nm for NPP. Therefore, they have difficulty in generating sum frequencies in the blue range of the spectrum. On the other hand, the inorganic optical materials such as KTP, LiNbO.sub.3, etc. can produce sum frequencies in the blue spectral range since their absorption edges are 400 nm or lower. However, the performance index of these inorganic optical materials for wavelength conversion is smaller than that of the organic optical materials by one figure or more. Similarly, the inorganic optical materials have a low wavelength conversion efficiency due to a low performance index when they are used to produce sum frequencies in longer wavelength ranges such as green and red spectral ranges.
U.S. Pat. No. 4,656,635, for example, shows a laser-diode-pumped solid-state laser in which a solid-state laser rod doped with a rare-earth material such as neodymium is pumped by a semiconductor laser. In order to obtain a laser beam having a shorter wavelength, the laser-diode-pumped solid-state laser includes a bulk single crystal of nonlinear optical material disposed in a resonator for converting the wavelength of a laser beam which is oscillated by the solid-state laser into the wavelength of a second harmonic or the like. It has been proposed to use a bulk single crystal of nonlinear optical material positioned in a resonator to convert the frequencies of a solid-state-laser-oscillated beam and a pumping beam into a sum frequency, as described in Applied Physics Letter Vol. 52, No. 2, 11 Jan. 1988, for example.
Since the conventional laser-diode-pumped solid-state lasers with the wavelength conversion capability employ inorganic nonlinear optical materials such as KTP. LiNbO.sub.3, etc.. their wavelength conversion efficiency is low. Specifically, when KTP is used to produce a sum frequency in the blue spectral range, the output of the sum frequency is only as intensive as 100 .mu.W at present because the performance index of KTP is very low.
With the low wavelength conversion efficiency, the efficiency with which the output energy is utilized is also low. If a highly intensive laser beam having a converted wavelength (i.e.. a shorter wavelength) is desired, then an expensive semiconductor laser of a very high output power of such as 200 mW or more is required as the pumping source. If such a high-output-power semiconductor laser is employed. a large and expensive system for radiating the heat from and hence cooling the semiconductor laser is also needed since a large amount of heat is produced by the semiconductor laser.
The wavelength conversion efficiency may be increased by using a large crystal which provides a long path for the laser beam, as the bulk single crystal of nonlinear optical material. However, it is technically difficult and highly costly to produce such a large crystal.
An increased wavelength conversion efficiency may also be achieved by using a nonlinear optical material having a larger nonlinear optical constant. Inorganic optical materials having nonlinear optical constants which are larger than that of KTP include LiNbO.sub.3, BNNB, and KNbO.sub.3 which is disclosed in Optics Letters. Vol. 13, page 137 (1988). for example. These inorganic nonlinear optical materials, however, fail to provide a stable wavelength conversion efficiency over a wide temperature range because the phase matching angle of these materials tends to shift due to a temperature change.
If the efficiency with which the solid-state laser is oscillated by the semiconductor laser is high, then the intensity of the solid-state-laser-oscillated laser beam that is applied to the nonlinear optical material becomes high, resulting in a wavelength-converted beam of a higher intensity. However, the conventional laser-diode-pumped solid-state laser has generally employed an array laser as the pumping source. Since the spectral line width of the array laser is as large as 10 nm, the efficiency with which the solid-state laser is oscillated is low and the energy utilization efficiency is also low.
There is known a single-transverse-mode, single-longitudinal-mode semiconductor laser as a semiconductor laser having a small spectral line width (which is normally as large as about 0.1 nm). The oscillation efficiency of the solid-state laser can be increased by controlling the temperature of the single-transverse-mode, single-longitudinal-mode semiconductor laser with a Peltier device so that the oscillation wavelength of the laser will match the absorption peak value of the solid-state laser. However, the presently available single-transverse-mode, single-longitudinal-mode semiconductor laser produces a lower output power than the array laser. To produce a wavelength-converted laser beam of a certain high intensity, the laser beams emitted by a plurality of single-transverse-mode, single-longitudinal-mode semiconductor lasers must be combined into a pumping laser beam. Such a system is costly to manufacture and low in reliability.